A variety of applications require the use of an alloy having a combination of high strength and high toughness. For example, ballistic tolerant applications require an alloy which maintains a balance of strength and toughness such that spalling and shattering are suppressed when the alloy is impacted by a projectile, such as a .50 caliber armor piercing bullet. Other possible uses for such alloys include structural components for aircraft, such as landing gear or main shafts of jet engines, and tooling components.
Heretofore, a ballistic tolerant alloy steel has been described having the following composition in weight percent:
______________________________________ C 0.38-0.43 Mn 0.60-0.80 Si 0.20-0.35 Cr 0.70-0.90 Mo 0.20-0.30 Ni 1.65-2.00 Fe Balance ______________________________________
The alloy is treated by oil quenching from 843.degree. C. (1550.degree. F.) followed by tempering. Tempering to a hardness of HRC 57 provides the best ballistic performance as measured by the V.sub.50 velocity. The V.sub.50 velocity is the velocity of a projectile at which there is a 50% probability that the projectile will penetrate the armor. However, when tempered to a hardness of HRC 57, the alloy is prone to cracking, shattering, and petal formation and the multiple hit performance of the alloy is severely degraded. To obtain the best combination of V.sub.50 performance and freedom from cracking, shattering, and petal formation, the alloy is tempered to a hardness of HRC 53. However, in order to provide effective anti-projectile performance at the lower hardness, thicker sections of the alloy must be used. The use of thicker sections is not practical for many applications, such as aircraft, because of the increased weight in the manufactured component.
Another alloy, with better resistance to shattering, cracking, and petal formation, has also been described. The alloy has the following composition in weight percent:
______________________________________ C 0.12-0.17 Cr 1.8-3.2 Mo 0.9-1.35 Ni 9.5-10.5 Co 11.5-14.5 Fe Balance ______________________________________
Although that alloy is resistant to cracking and shattering when penetrated by a high velocity projectile because of its good impact toughness, the alloy leaves much to be desired as an armor material since it has a peak aged hardness of HRC 52. Therefore, in order to provide effective anti-projectile performance, undesirably thick sections of the alloy must be used. As described above, the use of thick sections is impractical for aircraft.
In addition, an alloy has been described having the following composition, in weight percent:
______________________________________ C 0.40-0.46 Mn 0.65-0.90 Si 1.45-1.80 Cr 0.70-0.95 Mo 0.30-0.45 Ni 1.65-2.00 V 0.05 min. Fe Balance ______________________________________
The alloy is capable of providing a tensile strength in the range of 1931-2068 MPa (280-300 ksi) and a fracture toughness, as represented by a stress intensity factor, K.sub.Ic, of about 60.4-65.9 MPa.sqroot.m (55-60 ksi.sqroot.in.).
High strength, high fracture toughness, age hardenable martensitic alloys have been described having the following compositions in weight percent:
______________________________________ Alloy I Alloy II ______________________________________ C 0.2-0.33 0.2-0.33 Mn 0.2 max. 0.20 max. Si 0.1 max. 0.1 max. P 0.008 max. 0.008 max. S 0.004 max. 0.0040 max. Cr 2-4 2-4 Mo 0.75-1.75 0.75-1.75 Ni 10.5-15 10.5-15 Co 8-17 8-17 Al 0.01 max. 0.01 max. Ti 0.01 max. 0.02 max. Ce Trace-0.001 Small but effective amount up to 0.030 La Trace-0.001 Small but effective amount up to 0.01 Fe Balance Balance ______________________________________
Those alloys are capable of providing a fracture toughness as represented by a stress intensity factor, K.sub.Ic, of .gtoreq.109.9 MPa.sqroot.m (.gtoreq.100 ksi.sqroot.in.) and a strength as represented by an ultimate tensile strength, UTS, of about 1931-2068 MPa (280-300 ksi).
However, a need has arisen for an alloy having an even higher strength than the known alloys to provide improved ballistic performance and stronger structural components. It is known that fracture toughness is inversely related to yield strength and ultimate tensile strength. Therefore, the alloy should also provide a sufficient level of fracture toughness for adequate reliability in components and to permit non-destructive inspection of structural components for flaws which can result in catastrophic failure.